1. Field of the Invention
The present invention relates to a recording apparatus and its recording method used as an information output apparatus in word processors, copy machines, facsimiles and so on, and used as a printer connected to a host computer for outputting information from the host computer, and specifically to a recording apparatus and its recording method using a serial type recording head.
2. Description of the Prior Art
Characters and visualized images recorded on recording media such as recording sheets are digitized images generally formed by a set of pixels, each of which has individual gray level data. Each pixel is composed of a dot formed on the recording media by the recording head. As for recording heads for forming dots, a thermal transfer method and an ink jet recording method are widely known. Among them, an ink jet recording method has been widely used in recent years and it has many advantages in enabling relatively high fine-pitched dot formation and high-speed recording.
In expressing gray scale of recorded images with several gray levels, a dot density is assigned to each pixel according to these gray level data, and the dot pattern for each pixel is determined in accordance with the dot density. Definition forms of the dot patterns are categorized into two groups; one refers to a method that a plurality of dots are placed in an identical position in responsive to the dot density, and the other refers to a method that a designated dot pattern is developed with a plurality of dots in responsive to the dot density.
A dot pattern formed in the former method is relatively often used in a recording apparatus with an ink jet recording method. As for the former method, a multi-droplet method is known to be a method that, by forming dots by projecting a plurality of ink droplets ejected from an identical orifice onto a substantially identical position on the recording sheet, the dot density of the pixel can be controlled by changing the number of projected ink droplets.
Therefore, the multi-droplet method is effective for controlling the dot density in the ink jet recording method in which it is difficult to change the size of each ink droplet to a large extent, and effective particularly in a method in which ink droplets may be ejected by shock waves by a bubble generated in an ink fluid by thermal energy. This way of using thermal energy for ejecting ink droplets is effective for recording images with a high dot density and a great many gray-scale levels.
As a single pixel is formed by a plurality of ink droplets ejected from a single orifice in the multi-droplet method, in case that the amount of an ejected ink droplet changes from orifice to orifice and that there are orifices unable to eject ink droplets, shading in the recorded image may occur and the recorded image may contain stripe-noises (banding).
Above problems occur in case of forming a pixel by projecting dots with a designated dot pattern. In order to prevent those problems, in prior art recording head, the manufacturing process of the recording heads must be controlled precisely for reducing variations of the amount of ejected ink droplets from orifice to orifice. As a result, a manufacturing cost may increase and a productivity of recording heads cannot be attained to be a designated level.
As for a software-based method for preventing the shading in the recorded image, what has been known is a method for changing the number of ejected ink droplets so as to compensate the variation of the amount of an ejected ink droplets by image processing such as error diffusion method. However, the cost of the system may be often raised up by installing specific circuits for such image processing.
In addition, even if using such image processing methods, for example, in case that the variation of the amount of ejected ink droplets with respect to orifices changes while the recording apparatus is operated so long time, parameters for specifying the number of ejected ink droplets should be modified in needs, which may lead to the reduction of maintainability and usability of the system.